The present invention relates to improved processes for bleaching wood pulp and more particularly to processes for treating pulp slurry in oxygen-alkali media.
In a conventional chemical pulping process, wood chips are typically supplied to a digester vessel together with a cooking liquor to enable formation of a pulp slurry. In order to produce a pulp suitable for making paper, the pulp slurry formed in the digester is bleached, typically in a chlorine tower and is supplied to an extraction tower wherein delignified materials are removed from the pulp which is then subjected to one or more bleach stages. Commonly, the pulp slurry is washed upon leaving the chlorine tower and the extraction tower, and the filtrate separated from the pulp in the extraction tower washer may be recycled to the extraction tower and introduced into the lower reaches thereof. This recycled filtrate is frequently utilized to dilute the relatively high consistency (10-15%) pulp slurry as the same flows downwardly through the extraction tower. The pulp supplied from the extraction tower to a washer is typically of a relatively low consistency (2-6%) although the pulp leaving the washer is of a higher consistency, e.g. 10% or so. Bleaching is generally achieved in an upflow tower to which chlorine or chlorine dioxide is added. The pulp supplied to the extraction tower is treated under caustic or alkali conditions wherein sodium hydroxide, for example, or other alkali is added so that the delignified materials may be removed from or extracted from the pulp slurry.
In any pulp bleaching process, it is required that properties of the pulp such as burst, tear, viscosity, and freeness are not unduly compromised; yet, the process should be economical to operate in terms of water, steam, chemical consumption, etc. It is known to utilize oxygen in the extraction stage of a pulp delignification process in order to reduce the consumption of chemicals such as chlorine, chlorine dioxide, etc., or with the same consumption of chemicals, to increase delignification or brightness of each unit of pulp so treated. Oxygen-alkali pulp delignification processes have the advantage of tending to discharge fewer chlorine bearing compounds and consequently, tend to reduce the pollutants emitted from a pulp bleaching process. Such a process is described in an article entitled "New Opportunities for Reduction of Pollutants through Process Changes", Tappi Proceeding, March, 1981, which describes the use of oxygen-alkali extraction wherein oxygen and alkali are introduced into a thick stock pulp (8-12% consistency) prior to passage through an upleg tower. The chemical reactions commence in this upleg which is pressurized prior to passing the pulp slurry to a downflow extraction tower. Although the pollutant load and bleach chemical usage may be reduced by use of the process described in this article, significant and costly equipment is required in the form of a high shear mixer and a pressurized upleg which is typically 70-80 feet in height.
It is also known to add oxygen to an alkali extraction stage of a bleaching process as is described in an article entitled "Oxygen-Alkali Extraction; a Versatile Tool Towards a Simplified Bleaching Technique", 1982 International Pulp Bleaching Conference proceeding, pages 17-30. Oxygen is mixed with the pulp in a mixing device and is retained as a gas in the pulp matrix prior to passing with the pulp either upwardly through an upflow extraction tower or upwardly through a pressurized retention leg to enable passage of the pulp through a downflow extraction tower. Pulp consistencies of greater than 10% are utilized, and although this article teaches that the number of bleaching stages may be reduced, relatively expensive mixing devices are required and in some cases an upflow tower may also be necessary. Attempts to delignify pulp in oxygen-alkali environments wherein the capital investment so required is reduced are also described in an article entitled "Medium Consistency Oxygen Bleaching", Tappi Journal, April, 1980, pages 105-109, wherein pulp of 10-15% consistency is subjected to oxygen at relatively high pressures in an effort to reach the same delignification rates as would occur with high consistency pulp. In this process, there is no continuous gas volume in the reactor, and oxygen is transferred directly to cellulose fibers. However, an upflow reactor is required as are retention times of approximately 75 minutes. In addition, other systems for delignifying pulp in oxygen-alkali media are known as, for example, are shown in U.S. Pat. No. 4,198,266, which is assigned to the assignee of the present invention. Each of these prior art systems for delignifying pulp in oxygen-alkali solutions, however, require significant capital costs.
In other prior art processes for bleaching wood pulp, oxygen-alkali solutions are utilized to provide the environment for operation of particular equipment. U.S. Pat. No. 4,177,105 is exemplary of a process for delignifying pulp in oxygen-alkali solutions wherein a relatively complex set of rotating decks are arranged within a treating vessel to which oxygen, caustic and steam are introduced. The oxygen is introduced immediately above a dilution zone in the lower reaches of the treating vessel such that the oxygen passes countercurrently with the downflowing pulp. A pressure of approximately 140 psig is maintained in the vessel, and the reaction between pulp and oxygen is effected over a residence time of approximately 20-90 minutes. U.S. Pat. Nos. 3,832,276 and 3,951,737 also describe processes for delignifying wood pulp in oxygen-alkali environments wherein the pulp is mixed with oxygen and steam and supplied to a high pressure, pre-retention vessel or after such mixing is simply supplied to an upflow bleaching tower. These mixing devices are costly, and in conventional bleach plants, the effluent from the previous stage is at the top of a tower and additional pumps and piping is required to bring such pulp down to the bottom of an upflow tower. Consequently, not all of the processes and apparatus heretofore proposed for improving pulp bleaching processes is either inexpensive or readily adaptable to conventional, current bleaching equipment.
Processes for producing pulp in alkali media from raw materials are known as, for example, is described in U.S. Pat. No. 4,274,913. In this process, alkali cooking liquor and raw cellulose material are supplied to a high pressure vessel in which pressures of 70-350 psig are maintained. The raw material is impregnated with the cooking liquor as it flows downwardly through the vessel and is passed from a cooking zone to a cooling zone before the pulp is diluted in a dilution zone in the lower reaches of the pressure vessel. This reference teaches the addition of alkali and oxygen into the dilution zone through a complex nozzle and rotating scraping blade mechanism in order to establish a countercurrent flow between such oxygen and the downward flowing pulp. Pulp having a consistency of typically 4-10% is removed from the bottom of the pressure vessel. U.S. Pat. No. 4,295,926 also describes equipment for treating pulp with oxygen and essentially relates to a type of mixing device for adequately mixing oxygen gas with the pulp. The mixing device incorporates a plurality of members which pass through the pulp in a direction transverse to the direction of pulp travel and thereby, in accordance with the teachings of this reference, mix oxygen with the pulp. This mixing equipment is, however, relatively complex and costly and is not available at conventional pulping mills. In addition, relatively high pressures on the order of 100 psi are also required in order to effectively mix oxygen and pulp in accordance with the teachings of this reference.
As will be apparent from the foregoing discussion of prior art, processes and apparatus for treating wood pulp, all tend to require additional equipment such as reaction vessels, mixers, upflow legs, etc. In general, the use of an additional upflow leg in conjunction with a downflow extraction tower will require an additional downflow piping system as a typical chlorine stage is embodied in upflow towers, and some means is required to transport such pulp to the bottom of an upflow, pre-retention tower or leg when the same is utilized. In addition, a further pump and motor is required in order to so transport pulp through additional piping mentioned above. Furthermore, many current, conventional bleach plants simply do not have readily available space for the addition of such additional equipment as noted above. Consequently, although it may be theoretically feasible to retrofit equipment such as upflow legs, additional mixers, etc. to conventional processes, in fact, this is frequently difficult due to space limitations and generally is relatively expensive. Accordingly, there is a need for improved pulp bleaching processes wherein reduced chemical usages are obtainable yet do not require extensive capital equipment additions and their concomitant costs.